Molecular dynamics description of silver adatom diffusion on Ag(1 0 0) and Ag(1 1 1) surfaces

The self-diffusion processes of single adatoms on Ag(1 0 0) and Ag(1 1 1) surfaces have been studied using molecular-dynamics simulations and a many-body potential derived in the framework of the second-moment approximation to the tight-binding model. Our results for the (1 0 0) surface indicate that, although the migration energy for hopping is lower than that of the exchange mechanism, the exchange diffusion is higher than hopping diffusion for temperatures above 600 K. The migration energy for the hopping mechanism is in very good agreement with the experiment and the results of ab initio calculations. We also find that for the Ag(1 1 1) face the dominant mechanism is the hopping, which exhibit Arrhenius behaviour with two distinct temperature ranges, corresponding to two different migration energies. The diffusion in the high temperature region is mainly due to correlated jumps requiring an activation energy which is in excellent agreement with the experimental data. In addition the temperature dependence of the mean-square-displacements and the relaxations of both surface atoms and adatoms are presented and compared with previous studies.     

Kinetics of Oxidation for β-Sialon in Diphase β-Sialon/Al2O3 Composite

1.IntroductionSince198Os,non-isothermaltreatmenthasgrad-uallybecomeamainmethodforstudyingkineticprocess[1~6].Itisanefficienttoolwithadvantagesofwiderobservationtemperaturerange,moretime-savingandlesssamples['1"8J.SomeworkhavebeendoneontheoxidationkineticsofAl2O3/Sialon,O'-Sialonandcr+P-Sialonusingthemethodofisother-malki..ti.s[9~12].Howevernon-isothermalkineticsonSialonmaterialshasnotbeenreportedsofar.InthispapertheoxidationbehaviorofO-Sialonindiphaseg-Sialon/Al2O3compositehasbeenstudie…     

Particle size dependent Ag precipitation at different temperature and the resultant oxidation behavior of Cu-5Ag powders

Ag precipitation and the resultant oxidation behavior of Cu-5wt.%Ag powder of various particle sizes were investigated. During low-temperature aging (250 ℃), Ag precipitation along grain boundaries (GBs) in particle surface is closely dependent on particle size. The depth of Ag precipitation-free area (Ag-PFA) are 0 μm, 1.17 μm and 2.78 μm for S, M and L samples, respectively. While for high-temperature aging (550 ℃), the Ag precipitation was independent on particle size. This is ascribed to that Ag diffusion in particles at low temperature is dominated by both surface diffusion and GB diffusion, while at the higher temperature, the volume diffusion also becomes dominant. The formation of a continuous 3D-Ag network along GBs in particle surface can significantly improve the oxidation resistance of powders.     

Deformation response of grain boundary networks at high temperature

The deformation response of random grain boundary networks as a function of temperature and strain rate is explored using molecular dynamics atomistic simulations and an embedded atom method interatomic potential. We find that deformation at higher temperatures promotes both dislocation emission and grain boundary accommodation processes. The results allow estimating the activation energies and volumes for the deformation process. We find activation energy values for the deformation process similar to those for grain boundary diffusion and activation volumes consistent with an atomic shuffling mechanism. Our results suggest a picture of the deformation process as governed by the combination of the applied stress and thermally activated processes.     

Formation of intermetallic compounds in Mg–Ag–Al joints during diffusion bonding

In this study, we conducted the diffusion bonding of Mg and Al alloys using a 30-μm-thick pure silver foil interlayer at median temperatures between 390 and 490 °C. We obtained a multilayered structure across the Mg–Ag–Al joint: Mg/Mg(ss, Ag)/Mg₃Ag/MgAg/Ag/Ag(ss, Al)/Ag₂Al/Al. The silver diffusion barrier prevented the formation of brittle intermetallics between Mg and Al. Intermetallics identified at the joint interface include the more ductile types between Mg and Ag, ε-Mg₃Ag and β′-MgAg, and Ag and Al, δ-Ag₂Al. As the bonding temperature increased, Ag₂Al, followed by MgAg, favored the growth of Mg₃Ag IMC layer. The shear strength of the joints increased with the rising bonding temperature to a maximum value of 11.8 MPa at 470 °C. Fracture failure in the joints mainly occurred in the Ag₂Al layer. The formation mechanism for interfacial layers in the joints is believed to consist of four stages: (1) solid-solution formation, (2) Mg–Ag IMC formation, (3) Ag–Al IMC formation, and (4) growth of Mg–Ag and Ag–Al IMCs.     

Size dependency of self-diffusion and creep behavior of nanostructured metals

Grain boundary and bulk diffusion are two main governing processes of creep deformation in materials. Since the diffusion activation energy for nanostructured materials is lower than that for bulk, the diffusion is enhanced at the nano-scale, and nanosructured materials are expected to creep at lower temperatures and stresses. In this paper, a model has been developed to explain the effect of grain size on diffusion and creep behavior of nanostructures. Diffusion and creep phenomena have been shown to depend significantly upon size. Comparisons have been made with reported experimental results and related theoretical studies.     

等温时效对新型Sn-Ag基复合钎料显微组织和力学性能的影响

研究了等温时效对Sn-3.5Ag共晶钎料及其复合钎料的力学性能和显微组织变化的影响。为了弥补传统复合钎料制备和服役中强化颗粒容易粗化的问题, 制备了不同种类最佳配比的具有纳米结构的有机无机笼型硅氧烷齐聚物(POSS)颗粒增强的Sn-Ag基复合钎料。对钎焊接头在不同温度（125、150、175℃）下进行时效,通过SEM和EDAX分析了钎料与基板间金属间化合物层(IMC)的生长情况。结果表明, 经过不同温度时效,复合钎料钎焊接头界面处金属间化合物的生长速率比Sn3.5Ag共晶钎料慢, 复合钎料的IMC生长的激活能分别为80、97和77kJ/mol,均高于Sn3.5Ag共晶钎料。经过150℃时效1000h后,复合钎料钎焊接头的剪切强度分别下降了22%、13%和18%,下降幅度相当或明显小于Sn-3.5Ag钎料钎焊接头。      

Molecular dynamics simulation on diffusion of 13 kinds of small molecules in polyethylene terephthalate

Understanding the diffusion of migrants in polyethylene terephthalate (PET) and calculating the diffusion coefficients are very important for migration research. In this study, the diffusion coefficients of 13 kinds of small molecules with molecular weights ranging from 32 to 339 g/mol in amorphous PET are calculated based on molecular dynamics (MD) simulation. By comparison of diffusion coefficients simulated by MD simulation techniques, predicted by the Piringer model and by experiments, the accuracy of the Piringer model and MD simulation techniques for the estimation of diffusion coefficients of migrants in PET is evaluated. The MD simulation shows that D_simu is very close to D_exp, within one order of magnitude of the experimental diffusion coefficients except for a few samples. The possible reasons for the differences among D_simu, D_pred and D_exp are analysed from the molecular weight and temperature. The results show that the Piringer‐model‐predicted values at high temperatures overestimate significantly higher than that at lower temperatures. The activation energy is calculated by the Arrhenius equation, which shows the relationship between diffusion coefficient and temperature. It is shown that the MD simulation yields acceptable activation energy. The study suggests that MD simulation may be a useful approach to calculate the diffusion coefficients of small molecules in PET. Copyright © 2010 John Wiley & Sons, Ltd.     

Icosahedral Al–Mn phases grown in diffusion-limited conditions

Sequential deposition of Mn on a polycrystalline Al layer at 523–573 K was found to result in an anomalous icosahedral (i) Al–Mn phase, characterized by alterations of intensity- and linewidth ratios. The anomalies suggest a non-equilibrium atom distribution, related to the limited diffusion of Al through the already-formed layer of i-AlMn. The activation energy of Al diffusion was inferred from X-ray diffraction (XRD) data taken for different deposition temperatures.     

Molecular Dynamics Study on Interfacial Energy and Atomic Structure of Ag/Ni and Cu/Ni Heterophase System

The results of molecular dynamics calculations on the interfacial energies and atomic structures of Ag/Ni and Cu/Ni interfaces are presented. Calculation on Ag/Ni interfaces with low-index planes shows that those containing the (111) plane have the lowest energies, which is in agreement with the experiments. Comparing surface energy with interfacial energy, it is found the order of the interfacial energies of Ag/Ni and Cu/Ni containing the planes fall in the same order as solid-vapor surface energies of Ag, Cu and Ni. In this MD simulation, the relaxed atomic structure and dislocation network of (110)Ag||(110)Ni interface are coincident to HREM observations.     

Mg segregations at and near deformation-distorted grain boundaries in ultrafine-grained Al–Mg alloys

The formation of Mg segregations at and near deformation-distorted grain boundaries (GBs) in ultrafine-grained Al–Mg alloys is theoretically described as a process enhanced by stress fields of extrinsic dislocations existing at such GBs. The equilibrium Mg concentration profiles near low-angle and high-angle GBs containing extrinsic dislocations are calculated. The results of the calculations explain the experimental observations (reported in the scientific literature) of spatially inhomogeneous Mg segregations characterized by high Mg concentrations at and near GBs in ultrafine-grained Al–Mg alloys processed by severe plastic deformation.     

Effect of carbon on the diffusion of copper in different carbon-steels

Solid state diffusion is of great theoretical and practical importance. The tracer-technique can be applied to help solve many theoretical and practical problems. In this paper the thin layer method was used to study the diffusion of Cu64 in different carbon-steels. Experiments were performed at temperatures of 1173, 1193, 1213 and 1233 k in order to determine the diffusion coefficients of Cu64 in steels containing 0.2, 0.4, 0.6 and 0.8 wt% of carbon. From the results the effect of carbon content on the diffusion of Cu in steel was evaluated. Also the activation energies for the diffusion of Cu in the different specimens were calculated. This revised version was published online in November 2006 with corrections to the Cover Date.     

Micromechanical properties of grain boundaries and triple junctions in polycrystalline metal exhibiting grain-boundary sliding at 293 K

The mechanical properties of grain boundaries (GBs) and the development of grain-boundary sliding (GBS) were studied in the pure Zn using precision microindentation technique, optical, electron and atomic-force microscopy. Results have shown the different dependencies of the microhardness values on the indentation depth for GBs and individual grains. When the size of the plastic zone around the imprint was comparable to the grain size, GBs acted as barriers for dislocation sliding bands and twins. With applying the higher load, more grains were involved in the process of deformation, but microhardness did not increase. That was explained by the activation of GBS, leading to the relaxation processes. In its turn, the microhardness values measured at low loads in the vicinity to GBs and triple junctions (TJs) were higher than those measured in the grain interior. Thus, movement of the ensemble of defects to the GBs during microindentation is the activating factor for GBS in polycrystalline Zn. At the same time, during spreading of the deformation at low loads in the vicinity to GBs the activation of GBS was not observed.     

Prediction of Transport Properties of Liquid Ammonia and Its Binary Mixture with Methanol by Molecular Simulation

Transport properties of ammonia and of the binary mixture ammonia + methanol are predicted for a broad range of liquid states by molecular dynamics (MD) simulation on the basis of rigid, non-polarizable molecular models of the united-atom type. These models were parameterized in preceding work using only experimental vapor-liquid equilibrium data. The self- and the Maxwell-Stefan (MS) diffusion coefficients as well as the shear viscosity are obtained by equilibrium MD and the Green-Kubo formalism. Non-equilibrium MD is used for the thermal conductivity. The transport properties of liquid ammonia are predicted for temperatures between 223 K and 473 K up to pressures of 200 MPa and are compared to experimental data and correlations thereof. Generally, good agreement is achieved. The predicted self-diffusion coefficient as well as the shear viscosity deviates on average by less than 15 % from the experiment and the thermal conductivity by less than 6 %. Furthermore, the self- and the MS transport diffusion coefficients as well as the shear viscosity of the liquid mixture ammonia + methanol are studied at different compositions and compared to the available experimental data.     

Loading Rate Effects on Mechanical Response of Ag,Ni, Al and Cu at Nano‐Scale

Abstract: Molecular dynamics (MD) is now playing a unique as well as conspicuous role in probing in nano‐science and technology. In this paper, MD simulation of uniaxial tension of some face‐centred cubic (FCC) metals (namely, Ag, Ni, Al and Cu) at nano‐scale was carried out. The effects of different loading rates on the stress–strain curves at a constant temperature of 300 K were studied. The potential used in these simulations was Sutton–Chen, and Velocity Verlet formulation of Noise‐Hoover dynamic was also applied to have a constant temperature ensemble. The boundary condition was periodic. Eventually, MD simulations were carried out and it was concluded that by increasing the loading rate both maximum stress and strain at failure increase. The maximum engineering stress for Al, Ag, Cu and Ni can be rounded as 4, 9, 12 and 18 GPa, respectively, for all loading rates.     

Molecular Dynamics Simulation on Diffusion of Five Kinds of Chemical Additives in Polypropylene

Molecular dynamics (MD) simulation was used to investigate the diffusion behaviour of five additives [2,6‐di‐tert‐butyl‐4‐methylphenol (BHT), 2‐(2‐Hydroxy‐5‐methylphenyl)benzotriazole (UV‐P), 2,4‐Di‐tert‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl) phenol(UV‐327), 2‐(2H‐benzotriazol‐2‐yl)‐4‐(1,1,3,3‐tetramethylbutyl) phenol (UV‐329) and 2‐hydroxy‐4‐(octyloxy)benzophenone (UV‐531)] in polypropylene (PP) at the temperature of 293, 313 and 343 K. The diffusion coefficients were determined through Einstein relation connecting the data of mean square displacement at different times. The simulated diffusion coefficients were compared with that predicted by Piringer model and by experiments to evaluate the accuracy of MD simulation technique for estimating the diffusion coefficients of chemical additives in PP. Results showed that the simulated values were generally within one order of magnitude of the corresponding experimental values. The activation energies of additives were calculated by plotting the logarithm of diffusion coefficients versus the reciprocal of temperature according to Arrhenius equation. The activation energies calculated from MD simulation were also more closely to experimental values. Subsequently, the diffusion mechanism of additives inside PP was tentatively explored by analysing the interaction energy between diffusion molecules and polymer, free volume, molecular weight, size and shape, and the mobility of polymer chain. The movements of the additives in PP cell models at different simulation time suggested that for a long time, the additive molecules vibrate rather than hopping until they find the equal or larger transport channel to diffuse. It is demonstrated that the MD simulation may be a useful approach for predicting the microstructure and the diffusion coefficient of chemical additive with large molecular size and complex structure in polypropylene. Copyright © 2017 John Wiley & Sons, Ltd.     

Crystal growth study using combination of molecular dynamics and Monte Carlo methods

Molecular dynamics method although provides details of energies of the system as a function of time, is not suited to simulate the processes involving activation processes. Therefore, we attempted to combine the molecular dynamics and Monte Carlo methods. Using molecular dynamics, the energies of the system were calculated which were subsequently combined with Monte Carlo method using random numbers, epitaxial growth of (111) plane of copper, silver, and gold. While surface adsorption and surface diffusion for copper, silver, and gold were simulated by use of molecular dynamics method, the relation between the growth rate of thin films and the packing density of atoms were obtained using Monte Carlo simulation. Thus, by combining the results of the molecular dynamics method and the Monte Carlo method the growth process of thin films at elevated temperatures were obtained, which is too tedious to be calculated by molecular dynamics alone.     

Multiscale simulations of complex systems: computation meets reality

Multiscale simulations are evolving into a powerful tool for exploring the nature of complex physical phenomena. We discuss two representative examples of such phenomena, stress corrosion cracking and ultrafast DNA sequencing during translocation through nanopores, which are relevant to practical applications. Multiscale methods that are able to exploit the potential of massively parallel computer architectures, will offer unique insight into such complex phenomena. This insight can guide the design of novel devices and processes based on a fundamental understanding of the link between atomistic-scale processes and macroscopic behavior.     

Accelerated aging of Al/Ge and Al/Si thin film couples

The sheet resistances of Al/Ge and Al/Si thin film couples which had been heated in air or vacuum were measured. The annealing temperatures were kept below those at which any liquid phase is present. We find that at relatively low annealing temperatures the resistance of vacuum-annealed samples is higher than that of air- annealed samples. However, the observed peak resistance is higher for air than for vacuum annealing. Assuming a direct correlation between the sheet resistances and the diffusion profiles of the heated samples, we used the vacuum annealing data to obtain an activation energy E_a of 0.46 eV for diffusion in the Al/Ge structure and 0.70 eV for diffusion in the Al/Si couple. The results are discussed with reference to the electrical stability of silicon surface barrier detectors using an amorphous semiconductor/metal couple as the back contact.     

A study on the mechanism of serrated grain boundary formation in an austenitic stainless steel

Measurement of the activation energy for the formation of serrated grain boundaries (GB) has been carried out to understand its underlying formation mechanism in an AISI 316 stainless steel. The apparent incubation time necessary to initiate grain boundary serration was obtained at different aging temperatures, and the apparent activation energy for serration was carefully calculated from the Arrhenius relationship between incubation time and aging temperature. The activation energy for GB serrations in this alloy was measured to be approximately 148 ± 20 kJ mole⁻¹, which is consistent with the activation energy for lattice diffusion of carbon in γ-iron (142 kJ mole⁻¹). This result indicates that GB serration could be controlled essentially by the lattice diffusion of carbon to grain boundaries. Based on the through-thickness observation of serrated GBs, a straight boundary began to serrate from the surface at an early stage of the aging treatment, and then the serrated parts propagated throughout the entire grain boundary.